KR20090033269A - Optical filter matrix structure and associated image sensor - Google Patents

Abstract

The invention concerns an optical filter matrix structure composed of a set of at least two elementary (R, G, B) optical filters, one elementary optical filter being centered on an optimal transmission frequency, characterized in that it comprises a stack of n metallic layers (m1, m2, m3) and n appreciably transparent layers (d1, d2, d3) that alternate between a first nth metallic layer (m1) and an appreciably transparent layer (d3), the n metallic layers (m1, m2, m3) each having a constant thickness and at least one appreciably transparent layer having a variable thickness that determines the optimal transmission frequency of an elementary optical filter, n being a whole number equal to or greater than 2. Application to miniature image sensors.

Description

Optical filter matrix structure and associated image sensor

The present invention relates to an optical filtering matrix structure and to an image sensor comprising the optical filtering matrix structure according to the present invention.

The present invention can be particularly advantageously applied to manufacturing small sized image sensors such as, for example, miniature camera image sensors of mobile phones.

Acquiring images with electronic sensors is fully developed. The need to simplify the manufacture of sensors continues very strongly. Charge-coupled devices, commonly referred to as CCD sensors, are gradually being replaced by active pixel sensors in CMOS technology, which are commonly referred to as CMOS Active Pixel Sensors (APS).

One of the important problems that must be solved for an image sensor is to get a color. Starting with three colors taken from the visible spectrum (red, green, blue), it is possible to record and reproduce most colors.

Certain parts of the equipment begin by separating the three color bands and then directing the color bands towards the three image sensors. Others directly separate the collars on the single matrix surface of the detectors: a second type of said sensors relates to the present invention.

For the second type of sensors, two options are considered.

At several levels of the structure, using the fact that different colors do not penetrate to the same depth in the material (photosite) in which photons are converted to electrons, to create a very complex detection matrix;

Or adds a set of filters located as a matrix on the matrix surface of the detectors.

The second choice (adding sets of filters located as a matrix to the surface of the matrix of detectors) is mostly used. The most standard matrix is the matrix commonly referred to as the Bayer matrix.

An example of the Bayer matrix is shown in FIG. 1 as shown from the top. The Bayer matrix shown in FIG. 1 is a 2 × 2 (2 lines × 2 columns) matrix. From left to right, the filters of the row 1 line are the green filter and the red filter, respectively, and the filters of the row 2 line are the blue filter and the green filter, respectively.

Making such a filtering matrix is conveniently accomplished by using colored resins. In order to facilitate the preparation of the filtering matrix, resins which are photosensitive to ultraviolet irradiation are used, in which such non-isolated areas can be removed in a developing bath. For example, to produce the Bayer matrix according to the prior art, three layers of resin are deposited successively: one for green, one for red and one for blue. In each deposition, each of the resins is isolated and developed through a mask so that it remains only where it should be placed.

2 shows a simplified structure of APS CMOS from the prior art. The CMOS APS sensor is integrated with the photosensitive semiconductor element 1, the electrical interconnects 3, which are silicon, for example silicon, on the surface where the photosensitive region Zph and the electronic circuit E1 are formed and the electronic circuit E1. ) Includes a silica layer (2), a blue filter (B), a red filter (R), and a resin layer forming a green filter (V), a layer of resin (4), and a set of micro lenses (MC) connecting the two together. do.

Such sensor manufacturing techniques are currently well controlled. However, a disadvantage of these sensors is that it is impossible to filter the infrared rays. Therefore, behind the sensor, it is necessary to add a glass sheet provided with a multilayer interference filter for removing infrared rays.

Moreover, the resins are not dense and need to have a resin thickness greater than or close to 1 micron at present to obtain sufficient filtering effect. The size of the pixels of modern image sensors eventually approaches this size (typically 2 micrometers). This pixel size causes problems when the rays arrive with strong incidence on the surface of the sensor (edge of the image or strongly open object). Indeed, photons allowed to pass through the filter may end their progress at the photosite of the neighboring filter. This phenomenon therefore significantly limits miniaturization.

Colorized resins are also known to be easily homogenized. The filtering heterogeneity is therefore more pronounced because the pixels are small. This presents another disadvantage.

Moreover, although there are absorbent materials other than resins, if they are more absorbent, such materials pose a number of problems in the manufacture of those materials that are compatible with the simple manufacture of a matrix of integrated photosites, and such manufacturing is too expensive. It takes

The present invention does not have the disadvantages mentioned above.

Indeed, the invention relates to an optical filtering structure consisting of a set of at least two basic optical filters, wherein one basic optical filter is centered at an optimal transmission frequency, and the optical filtering structure comprises a first metal layer and an nth substantially And a stack of n metal layers and n substantially transparent layers alternated between transparent layers, each of the n metal layers having a constant thickness, at least one substantially transparent layer having a variable thickness, The thickness sets the optimum transmission frequency of the basic optical filter, where n is an integer greater than or equal to two.

According to a further feature of the invention, where n = 2 and only one substantially transparent layer has a variable thickness, the substantially transparent layer having a variable thickness is substantially transparent located between the first metal layer and the second metal layer. Layer.

According to another additional feature of the invention, the two layers which are n = 3 and which are substantially transparent have a variable thickness, and wherein the substantially transparent first layer having the variable thickness is located between the first metal layer and the second metal layer. The substantially transparent second layer, which is a substantially transparent layer and has a variable thickness, is positioned between the second metal layer and the third metal layer, and the overthickness resulting from the thickness change of the substantially transparent first layer is substantially Substantially laminated with the transient thickness resulting from the change in thickness of the transparent second layer.

According to another additional feature of the invention, the basic optical filters are located as a matrix.

According to another additional feature of the invention, the matrix is a Bayer matrix for filtering three colors of red, green and blue.

According to another additional feature of the invention, the metal layers are silver (Ag).

According to another additional feature of the invention, the material from which the substantially transparent layers are made is selected from titanium dioxide (TiO ₂ ), silica (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), hafnium oxide (HfO ₂ ).

The invention also relates to an optical sensor, which comprises an optical filtering structure and a photosensitive semiconductor substrate on which the optical filtering structure is deposited, the optical filtering structure being a structure according to the invention, the first metal layer of which is a And deposited on the first side.

The invention is an optical sensor comprising an optical filtering structure and a photosensitive semiconductor substrate on which the optical filtering structure is deposited, the optical filtering structure being a structure according to the invention, the optical sensor comprising a barrier layer, the first layer of the barrier layer A surface is deposited on the first surface of the semiconductor substrate and the second surface of the barrier layer is in contact with the first metal layer of the optical filtering structure.

According to a further feature of the invention, the barrier layer is made of the same material as the material forming the substantially transparent layers of the optical filtering structure.

According to a further feature of the invention, the barrier layer is partly or wholly electrically conductive.

According to a further feature of the invention, the material of the barrier layer is indium doped tin oxide (ITO) in electrically conductive portions and silica (SiO ₂ ) or silicon nitride (Si ₃ N in electrically nonconductive portions). ₄ ).

 According to a further feature of the invention, the photosensitive region and the electronic components are formed on the first side of the photosensitive semiconductor.

According to another additional feature of the invention, the photosensitive region and the electronic components are formed on the second side of the photosensitive semiconductor, opposite the first side.

Multilayer filters consisting of altered transparent layers and metal layers are known for producing structures having a photonic forbbiden band, commonly referred to as Photonic Band Gap (PBG). U. S. Patent No. 6,262, 830 discloses a metal dielectric transparent structure having a photonic suppression band.

Metal-dielectric transparent structures having a photonic suppression band disclosed in US Pat. No. 6,262,930 consist of superimposing a plurality of transparent dielectric layers separated by thin metal layers and having a thickness close to half the wavelength. The thickness of each dielectric layer or metal layer is uniform. These structures are designed to pass certain frequency bands and block others. A disadvantage of this structure is that some of the light is absorbed where it contains areas where the structure is desired to be transparent.

In a manner unexpected to one skilled in the art, the present invention appears to produce an optical filtering structure adapted for transmission at any particular frequency while varying the thickness of only one or two transparent layers while all other layers are of constant thickness. . As shown in the cross-sectional view, the transparent layer (s) having varying thicknesses vary step by step according to their position in the matrix.

Advantageously, the optical filtering structure according to the invention, for example Bayer matrix, can be such that all the basic optical filters of the matrix have a thickness less than the minimum of the useful wavelengths.

Other features and advantages of the present invention will become apparent by reading the preferred embodiment of the present invention with reference to the accompanying drawings.

1 shows a plan view of a Bayer matrix according to the prior art as already described.

Fig. 2 shows a cross-sectional view of a CMOS APS sensor that has already been described and which is different from the prior art.

3 shows a top view of an optical filtering matrix according to the invention.

4A and 4B show cross-sectional views along two different axes of an optical filtering matrix structure according to a first embodiment of the invention.

5 shows the performance of optical filtering of an optical filtering matrix structure according to a first embodiment of the invention.

6A and 6B show cross-sectional views along two different axes of an optical filtering matrix structure according to a second embodiment of the present invention.

7 shows optical filtering performance of an optical filtering matrix structure according to a second embodiment of the present invention.

8 shows a sensor block cross-sectional view using an optical filtering matrix structure according to a second embodiment of the invention.

9 shows a first alternative sensor according to the invention.

10 shows a first alternative sensor according to the invention.

In all drawings, the same reference numerals represent the same components.

3 shows a plane for the optical filtering matrix structure of the present invention. As shown from above, the optical filtering matrix structure of the present invention has a geometric shape obtained by repeating the basic structure from the prior art shown in FIG. Filtering cells (R, V, B) for selecting colors of red, green and blue are placed next to each other.

4A and 4B illustrate a cross sectional view along two different axes of an optical filtering matrix structure according to a first embodiment of the invention. Referring to FIG. 3, FIG. 4A is a sectional view along axis AA of FIG. 3, and FIG. 4B is a sectional view along axis BB of FIG. 3. The axis AA cuts the optical filters V of the green diagonally. The axis BB is an axis perpendicular to the axis AA and is continuous blue (B), green (V), red (R), green (V), blue (B), green (V) on the outside of the diagonal. ), And cut optical filters such as red (R).

The image sensor comprises a photosensitive semiconductor element 1, for example silicon, in which a first metal layer m1, a first transparent layer d1, a second metal layer m2 and a second transparent layer d2 are continuously Is placed. The metal used to make the metal layers m1 and m2 is, for example, silver (Ag), and the metal used to make the transparent layers d1 and d2 is, for example, a dielectric, for example titanium dioxide (TiO _2). May be).

Layer d1 is an adjustment layer, in which the change in thickness of the adjustment layer changes the different transmission wavelengths of the filter, all other layers having a constant thickness. The thickness change of the layer d1 is thus adapted to the selective transmission of the blue color (thickness e1), the green color (thickness f1) and the red color (thickness g1).

In certain cases where the metal layers m1, m2 are layers of silver (Ag) and the transparent layers (d1, d2) are titanium dioxide (TiO ₂ ) layers, the thicknesses of layers m1, m2 and d2 are the same, For example 27 nm, 36 nm and 41 nm, respectively, and the thickness of the layer d1 varies between 50 nm and 90 nm; i.e. 52 nm for blue, 70 nm for green and 87 nm for red. .

The thickness of the layers will take different values for different materials. As a non-limiting example, the metal layers can be made of Ag, Al, Au, Nb, Li, and the transparent layers are TiO ₂ , ITO, SiO ₂ , Si ₃ N ₄ , MgF ₂ , SiON, Al ₂ O ₃ , HfO ₂ can be produced. In general, the thicknesses of layers m1, d1, m2, d2 are calculated as algorithms for multi-layer filter calculations.

It should be noted that in FIGS. 4A and 4B (and this is true for all other drawings as well) the thickness of the layer is intentionally enlarged to better represent the thickness change.

5 shows optical filtering capabilities of a filtering matrix structure according to a first embodiment of the invention. These curves are shown in FIG. 5, that is, the transmission curve C1 for the blue color (for blue filters), the transmission curve C2 for the green color (for green filters), and the red filters (for red filters). ) Transmission curve (C3) for red color. The curves C1, C2, C3 show the transmission coefficients of the matrix structure, expressed as a percentage according to the wavelength [lambda] expressed in nm.

It should be noted that not only the good quality of the transmission results obtained with only four layers (transmission achieves substantially 70%), but also good rejection of infrared wavelengths above 900 nm can be obtained.

Moreover, the optical filters exhibit one transmission peak between near ultraviolet (400 nm) and infrared (1100 nm). This is advantageous when compared to the dielectric structures of the prior art, which already have parasitic transmission in near infrared (> 800 nm).

6A and 6B show cross-sectional views along two different axes of an optical filtering matrix structure according to a second embodiment of the invention. Referring to FIG. 3, FIG. 6A is a cross sectional view along axis AA, and FIG. 6B is a sectional view along axis BB.

The image sensor comprises a photosensitive semiconductor element 1, for example a semiconductor, in which three metal layers m1, m2, m3 and three transparent layers d1, d2, d3 are alternately arranged, and the metal layer m1 ) Is in contact with the semiconductor element 1. The metal layers m1-m3 are silver (Ag), for example, and the transparent layers d1-d3 are titanium dioxide (TiO ₂ ), for example.

The layers d1 and d2 are adjustment layers, the thickness change of which sets different transmission wavelengths of the filter and all other layers have a constant thickness. The excess thickness resulting from the thickness change of layer d1 coincides with the excess thickness resulting from the thickness change of layer d2 (excess thickness builds up). Layer d3 is an antireflective layer. The change in thickness of the layers d1 and d2 is thus adapted for the selective transmission of different colors:

The thickness e1 of the layer d1 and the thickness e2 of the layer d2 are associated for selective transmission of the blue color;

The thickness f1 of layer d1 and the thickness f2 of layer d2 are associated for selective transmission of the green color;

The thickness g1 of the layer d1 and the thickness g2 of the layer d2 are associated for selective transmission of the red collar.

In the particular case of metal layers made of silver (Ag) and transparent layers of titanium dioxide (TiO ₂ ), the thicknesses of the layers m1, m2, m3 and d3 are for example 23 nm, 39 nm, 12 nm and 65 nm, respectively, and the layer The thickness of (d1, d2) is comprised between 50 nm and 100 nm, ie:

 d1  d2 blue  52 nm  51.4 nm Green  70 nm  73.5 nm Red  87.5 nm  96 nm

The transmission spectrum resulting from this matrix structure is shown in FIG. 7. The curves D1, D2, D3 represent the transmission coefficient T of the matrix structure, expressed as a percentage according to the wavelength [lambda] expressed in nm. At central wavelengths corresponding to three desired colors (red, green, blue), transmission is comprised between 60 and 70%.

8 shows a cross-sectional view of an enhancement of an optical filtering matrix structure according to a second embodiment of the invention. The matrix structure is here provided with a set of micro lenses MC, which focus the light L at the photosite. As is known, the micro lenses MC are disposed on the planarization layer P. FIG.

In addition to the microlenses MC and the planarization layer P, which have already been described with reference to FIGS. 6A and 6B, the matrix structure of the invention comprises a barrier layer b, which is a semiconductor ( 1) is protected from the metal layer m1. The barrier layer b prevents the semiconductor 1 from being contaminated by the metal layer m1 (prevents contamination by metal ions moving to the semiconductor). For example, where the metal is silver (Ag), the desired protection can be achieved with a silica barrier layer or a layer of tin oxide (ITO) (indium tin oxide) doped with indium. The thickness of the silica layers or the thickness of the ITO layer is for example equal to 10 nm.

Barrier layer (b) may be non-conductive (this is the case for silica (SiO ₂ ) and silicon nitride (Si ₃ N ₄ )), conductive (this is the case for ITO), or partially conductive. . When conductive, the barrier layer b can advantageously be used as an electrode at the surface of the semiconductor 1. Advantageously, where the barrier layer is an ITO barrier, ITO can be used to produce transparent layers of the structure, which is transparent. Layers (b, d1, d2, d3) are ITO and layers (m1, m2, m3) are Ag. Two materials (Ag and ITO) are sufficient for producing the structure according to the invention in that the photosensitive semiconductor is protected from contact with the metal. It is also possible to replace silver with a metal alloy that has good index characteristics but is less contaminant to semiconductors.

9 and 10 show two alternative optical sensors according to the invention.

9 shows an optical sensor, wherein light reaches the surface of the photosensitive semiconductor on which the photosensitive region Zph and the electronic circuit E1 are formed. In this type of optical sensor, light must avoid the metal interconnects 3. The optical filters of the present invention are much thinner overall than prior art filters. It is advantageous to arrange the filters as close as possible to the photosensitive regions Zph between the interconnects 3. Therefore, compared with the corresponding structure of the prior art (compared with FIG. 2), the optical sensor has a better insensitivity to incidence of light.

FIG. 10 shows an optical sensor, where light reaches the surface of the photosensitive semiconductor opposite to the surface on which the photosensitive region Zph and the electronic circuit E1 are formed. The optical filtering matrix of the present invention is very easily adapted to this type of sensor. Here, the optical sensor of the present invention advantageously has a thinner thickness than the corresponding optical sensor of the prior art. Moreover, in certain applications, the proximity of the optical filters advantageously allows the micro-lenses MC to be pressed.

According to a particular embodiment, the sensor shown in FIG. 10 comprises a barrier layer (b) between the semiconductor (1) and the first metal layer (m1), the barrier layer having an electrically conductive region (k1) and an electrically insulating region ( k2) is provided. The conductive region k1 and the insulating region k2 allow electrical contact to be established at the desired positions.

The two specific structures shown in FIGS. 9 and 10 advantageously have a transmission spectrum that hardly changes with the incident angle of incidence of light. Thus, for example, a green filter (TiO ₂ / Ag) having a filter bandwidth of 90 nm changes its average wavelength to substantially 20 nm when the incident angle is changed from 0 ° to 40 °. Relatively, this change is 38 nm for the multilayer filter (SiO ₂ / TiO ₂ ) of the prior art.

Another advantage of the filtering structures of the present invention is that they can be made of materials such as silver and ITO, which are electrical conductors, so that the filter can act as an electrode, and these electrodes are connected to the underlying circuits E1 and Zph. There may be several contact points.

The technique of manufacturing the filtering structure and sensors of the present invention is simple and utilizes conventional manufacturing processes in the microelectronics art.

Optionally, the transparent layer and the metal layer are manufactured by vacuum sputtering, but other techniques may also be used, such as, for example, vacuum deposition. By knowing the rate of deposition, control of the thickness is achieved.

A method for manufacturing an exemplary optical filtering structure having a single transparent layer of varying thickness will be described later.

A protective layer (SiO ₂ ), a metal layer (silver) and a transparent material layer (ITO) are successively deposited on the photosensitive semiconductor substrate. Next two steps of photolithographic etching are performed. Masking of areas that should not be etched is made of resin. Preferably, the etching is performed as reactive ion etching (eg under chlorine + HBr gas for etching ITO). The timing of stopping the etching is determined by an interferometer. As a non-limiting example, from an ITO layer with a thickness of 90 nm, an ITO thickness of 70 nm can be obtained for green, a 50 nm thickness for blue, and an initial thickness of 90 nm for red. maintain. Each of the silver (Ag) layer and the ITO layer having a constant thickness is deposited successively.

A method for manufacturing an exemplary optical filtering structure with two transparent layers of varying thickness is described below.

The method is almost identical to that described above for a single transparent layer of varying thickness:

Deposit three first layers (protective SiO ₂ layer, first Ag layer, first ITO layer);

Perform two photolithography-etching steps to make transparent steps of the first ITO layer;

Deposits two next layers (second Ag layer, second ITO layer);

Perform two new photolithography-etching steps to make transparent steps of the second transparent layer;

Deposit the last two layers (Ag, ITO) of constant thickness.

As already mentioned previously, it is advantageously provided by the present invention that manufacturing a protective layer can be more complicated than a simple dielectric layer. For example, if it is intended to use a rudimentary filter as the conductive electrode, it is necessary to replace the insulating protective layer with the conductive protective layer at the positions where the protective layer is in electrical contact with the photosensitive semiconductor.

The manufacture of such a layer with two materials, for example SiO ₂ for producing insulating regions and ITO for producing conductive regions, is carried out in four steps:

Depositing a SiO ₂ layer;

Photolithography and etching SiO ₂ where it is intended to place ITO;

Depositing an ITO layer slightly thicker than the SiO ₂ layer;

ITO is removed by mechano-chemical planarizaton to the surface of SiO ₂ .

The present invention can be applied to the field of image sensors.

Claims (14)

An optical filtering structure consisting of a set of at least two basic optical filters (R, V, B), wherein the basic optical filter is centered at an optimal transmission frequency, The optical filtering structure comprises n metal layers m1, m2, m3 and n substantially transparent layers d1, d2, d3 alternated between a first metal layer m1 and an nth substantially transparent layer d3. ), Each of the n metal layers (m1, m2, m3) has a constant thickness, and the at least one substantially transparent layer has a variable thickness that sets the optimal transmission frequency of the basic optical filter. And n is an integer greater than or equal to two. The method of claim 1, n = 2, the single substantially transparent layer has a variable thickness, and the substantially transparent layer with the variable thickness is a substantially transparent layer located between the first metal layer m1 and the second metal layer m2, Optical filtering structure. The method of claim 1, n = 3 and the two substantially transparent layers have a variable thickness, and the substantially transparent first layer having the variable thickness is located substantially between the first metal layer m1 and the second metal layer m2. The transparent layer, the substantially transparent second layer having a variable thickness is located between the second metal layer and the third metal layer m3, and the overthickness resulting from the thickness change of the substantially transparent first layer is substantially And laminated with an excessive thickness resulting from a change in thickness of the transparent second layer. The method of claim 1, wherein The basic optical filters (R, V, B) are arranged as a matrix. The method of claim 4, wherein The matrix is an optical filtering structure, which is a Bayer matrix for filtering three colors of red, green and blue. The method of claim 1, wherein And the metal layers are silver (Ag). The method of claim 1, wherein The material for making the substantially transparent layers is selected from titanium dioxide (TiO ₂ ), indium doped tin oxide (ITO), silica (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), hafnium oxide (HfO ₂ ), Optical filtering structure. An optical sensor comprising an optical filtering structure and a photosensitive semiconductor substrate 1 on which an optical filtering structure is deposited, the optical filtering structure being a structure according to any one of claims 1 to 7, wherein the first metal layer m1 Is deposited on the first side of the semiconductor substrate (1). An optical sensor comprising an optical filtering structure and a photosensitive semiconductor substrate 1 on which an optical filtering structure is deposited, The optical filtering structure is a structure according to any one of claims 1 to 7, wherein the optical sensor comprises a barrier layer (b), the first side of the barrier layer being deposited on the first side of the semiconductor substrate (1). And the second side of the barrier layer is in contact with the first metal layer (m1). The method of claim 9, The barrier layer is made of the same material as the material from which the substantially transparent layers of the optical filtering structure are made. The method according to claim 9 or 10, The barrier layer (b) is partially or wholly electrically conductive. The method of claim 11, The material of the barrier layer is indium doped tin oxide (ITO) in electrically conductive portions and silica (SiO ₂ ) or silicon nitride (Si ₃ N ₄ ) in electrically non-conductive portions. The method according to any one of claims 8 to 12, The photosensitive region (Zph) and the electronic components (El) are formed on the first side of the photosensitive semiconductor. The method according to any one of claims 8 to 12, The photosensitive region (Zph) and the electronic components (El) are formed on the second side of the photosensitive semiconductor (1) opposite the first side.

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